The present invention relates to a power transmission apparatus of a working machine having a clutch for transmitting engine power to a drive wheel of the working machine, particularly to a power transmission apparatus of a self-propelled working machine such as a lawn mower and a grass mower in which engine power is transmitted to a clutch through a belt and a pulley for disconnecting and connecting the power with a drive wheel.
Japanese Laid-Open Patent Publication No. 61-55412 or U.S. Pat. No. 4,117,652 discloses a power transmission apparatus having a clutch provided at a rear axle to which engine power is transmitted through a belt device for driving a rear wheel of a working machine.
In the prior art, an engine mounted on a body of a self-propelled lawn mower has an output shaft extending vertically downward and a cutter blade and a drive pulley are fitted to the output shaft. On the one hand, a rear axle of the lawn mower is provided with a clutch for disconnecting and connecting engine power with the rear wheel. An input shaft for transmitting engine power to the clutch extends vertically in a transmission case holding the rear axle supported rotatably. A driven pulley is attached to the input shaft. A belt is wound round the drive pulley and the driven pulley. The driven pulley has a fixed sheave and a movable sheave which is movable in an axial direction of the input shaft toward or away from the fixed sheave to change an effective diameter of the pulley for speed change. The engine power is transmitted to the driven pulley from the drive pulley through the belt to rotate the input shaft. When claws of the clutch are engaged so that the clutch connects, rotation of the input shaft is transmitted to the rear axle to rotate the rear wheel and lawn mower can be self-propelled.
In the above-mentioned power transmission apparatus, when the clutch connects, power of the drive pulley acts on the clutch abruptly through the input shaft and also reaction force due to rapid drive of the stopped wheel acts on the clutch so that the clutch is burdened with a large load. Therefore, it is required to raise rigidity of the clutch to cause enlargement and weigh increasing of the apparatus. As a result, working and operation of the lawn mower are deteriorated. Further, the above-mentioned reaction force influences the belt wound round both the pulleys too, so as to give the belt an excessive tension to shorten its life.
In a lawn mower having the power transmission apparatus mounted, because on connection of the clutch, engine power is transmitted to the rear wheel abruptly and the rear wheel having been stopped is driven rapidly at a high rotative speed, grasses are fallen down or torn and sometimes the ground is scraped by the rear wheel, so that finished state of the lawn is not necessarily good.
An object of the present invention is to overcome the above-mentioned difficulties and provide a power transmission apparatus of a working machine in which on connection of the clutch, an abrupt transmission of engine power is avoided and the engine power can be transmitted gradually.